The condensation of 1,4-dihalobutene-2 such as 1,4-dichlorobutene-2, with lower alkyl malonic esters such as dimethyl malonate is widely used in industry to prepare various vinylcyclopropane derivatives such as dimethyl 2-vinylcyclopropane-1,1-dicarboxylate which has utility in a wide variety of industrial applications.
The starting material for this condensation, 1,4-dichlorobutene-2 as a commercial product, is usually composed of three isomeric dichlorobutenes, trans-1,4-dichlorobutene-2, cis-1,4-dichlorobutene-2 and 3,4-dichlorobutene-1. Trans-1,4-dichlorobutene-2 is the preferred starting material as the stereochemistry of the intermediate (I) is such that the desired dimethyl 2-vinylcyclopropane-1,1-dicarboxylate is the exclusive product. This is illustrated in Mechanism I below: ##STR1##
Cis-1,4-dichlorobutene-2 gives two products, dimethyl 2-vinylcyclopropane-1,1-dicarboxylate and dimethyl cyclopent-3-ene-1,1-dicarboxylate in nearly equal amounts as shown in Mechanism II below. ##STR2##
It is nearly impossible to separate dimethyl cyclopent-3-ene-1,1-dicarboxylate from the desired dimethyl 2-vinylcyclopropane-1,1-dicarboxylate by any reasonable means. In the described condensation reaction the third isomer, 3,4-dichlorobutene-1 gives only useless elimination products.
Fractional distillation of the three dichlorobutene isomers can readily be accomplished, but this is an expensive process and leads to the additional difficulty and cost of disposing of 3,4-dichlorobutene-1 and cis-1,4-dichlorobutene-2.
Efforts have been made in terms of isomerizing cis-1,4-dichlorobutene-2 to trans-1,4-dichlorobutene-2, but these methods have met with only moderate success.
Heterogeneous iron, tin and copper compounds as well as onium salts have been reported in the literature as dichlorobutene isomerization catalysts. A typical process employing a copper catalyst is disclosed in U.S. Pat. No. 2,911,450. However, such processes are not completely satisfactory as they either have proven to be ineffective in some instances or have given equilibrium mixtures of all three dichlorobutenes.
The use of thiols as cis-to-trans-olefin isomerization catalysts has also been reported in the literature. See, W. G. Niehaus, Jr., Bioorg. Chem., 3(3), 302-10 (1974) and C. Walling, et al., J. Amer. Chem. Soc., 81, 1144-8(1959) as has hydrogen bromide-catalyzed isomerization. See N. P. Neureiter, et al., J. Amer. Chem. Soc., 82, 5354-8 (1960). However, the thiol-catalyzed and hydrogen bromide-catalyzed isomerization of olefins typically leads to an equilibrium mixture of approximately 80% trans- and 20% cis-olefin. This appears to be true regardless of whether the starting olefin is cis or trans. See C. Walling, et al. Ibid.
It would be highly desirable, therefore, if an improved process could be developed which would permit an efficient cis-to-trans isomerization of 1,4-dichlorobutene-2 so that a high trans (&gt;90%) mixture could be obtained from the usual commercial mixture of 1,4-dichlorobutene-2, which normally has a trans/cis ratio of 77/23, or from other mixtures having even lower trans content without any of the attendant disadvantages of the prior art.
It would also be highly desirable to provide a product with a high content of trans-1,4-dichlorobutene-1 and being substantially free from the other two isomers, cis-1,4-dichlorobutene-2 and 3,4-dichlorobutene-2, which in the described condensation reaction with malonic esters either give approximately equal amounts of the desired dimethyl 2-vinylcyclopropane-1,1-dicarboxylate and the unwanted dimethyl cyclopent-3-ene-1,1-dicarboxylate or in the case of the isomeric 3,4-dichlorobutene-1 only useless elimination products.